My counsel is to use caution when tapping/slapping cars. People can get very strange and angry over such actions. You never know who is crazy, drunk, stoned, or all three. In fact, I'm guessing if they did something stupid enough to provoke a response like slapping their car, the probabilites are better than average that they will one of the above. Then, if they decide to rub you out for touching their car, all they have to do is turn the wheel and hit the gas....